Sewer lines on the island are often as old as the houses they serve, and what is above them is usually worth protecting.
Old lines, and what gets into them
Where a lateral is original clay or early cast iron, the joints weep. On a barrier island the groundwater that gets in is brackish and carries fine sand, which settles in low spots and gradually silts the line. That is a different problem from a blockage and it needs a different fix.
Mature gardens and old trees
Sullivan’s Island lots have established planting and substantial trees whose roots reach much further than the canopy suggests. Roots find weeping joints, and trenching to reach a line risks the tree as well as the garden.
Digging here is awkward
A high water table means a trench fills as fast as you dig it and needs proper shoring. Putting the ground back afterwards becomes its own job, which is why lining is so often the better answer on this island.
Camera before anything else
Silting, roots, a sag, a crack and a collapse cost very different amounts to put right. You should be able to see the problem you are paying to solve.